In 2007 divers mapping the underwater caves on Mexico's Eastern Yucatán Peninsula, about 12 miles north of the city of Tulum, stumbled into a very large chamber.
"The floor disappeared under us, and we could not see across to the other side," Alberto Nava of Bay Area Underwater Explorers, told Discovery News.
They named the pit Hoyo Negro (Spanish for Black Hole).
Here, divers Susan Bird and Alberto Nava explore Hoyo Negro.
About two months later, armed with powerful underwater lights, Nava and other diver colleagues reached the floor of Hoyo Negro at about 170 feet.
They found themselves in a bell-shaped structure 200 feet in diameter, whose center was littered with large boulders stacked on top of each other.
This picture shows the immensity of the chamber and the complexity of the boulder strewn bottom.
"As our eyes got accustomed to the environment, we started to notice large animal bones resting at the bottom and on the walls of the pit," Nava said.
Overall, international researchers led by anthropologist James Chatters identified the remains of more than 26 large mammals. They included a gomphothere, an extinct elephant-like creature, which was dated to around 40,000 years ago, saber-toothed cats and giant ground sloths, which were largely extinct in North America by 13,000 years ago. Extant species included puma, bobcat, coyote, Baird's tapir, collared peccary and a bear.
In this picture, Nava inspects a forelimb of an extinct Shasta ground sloth, a species not previously found so far in the Americas.
One diver then spotted a human skull resting on the top of a small ledge. The small cranium lay upside down and rested on the left humerus (upper arm bone) with other remains nearby.
"It had a perfect set of teeth and dark eye sockets looking back at us. We could see the rest of the upper torso spread to the left and down on the ledge," Nava said.
The small skeleton, named "Naia" by the dive team (meaning water nymph in Greek) belonged to a female measuring only 4'10" tall. She is estimated to have been between 15 and 16 years old at the time of her death.
"When Naia and the animals entered the cave, the near-surface tunnels were dry, so they walked in from a ground-level entrance, probably a sink hole. They walked a considerable distance, as much as 600 meters," Chatters said.
He speculates that Naia, and the larges animals in particular, were drawn by a large, ephemeral pool of water in the bottom of Hoyo Negro.
"Yucatan was a dry place back then. Walking in the dark, they fell into the deep pit, from which there was no exit," Chatters said.
Using photography, videography, three dimensional modeling and sampling, researchers studied the skeleton without removing it from its watery grave.
The human remains were dated between 13,000 and 12,000 years, making the skeleton one of the six oldest humans yet found in the Americas, certainly the oldest and most complete.
Here, divers Susan Bird and Alberto Nava transport the Hoyo Negro skull to an underwater turntable so that it can be photographed for 3d modeling.
Naia does not feature the broader and rounder skulls of today's Native Americans. She bore a long and high cranium, a pronounced forehead, a low and flat nose. Her teeth projected outward from her small face.
Such different faces, skulls and teeth have led speculations that prehistoric Americans might represent an earlier migration from Southeast Asia or even Europe via a submerged land mass where the Bering Sea is now. But mitochondrial DNA testings -- maternally inherited DNA -- suggest a different scenario.
Here, diver Susan Bird works at the bottom of Hoyo Negro and carefully brushes Naia's skull.
DNA testing carried out from Naia's upper right third molar indicates Paleoamericans and Native Americans descended from the same land in Beringia, a now partially submerged landmass including parts of Siberia, Alaska and the Yukon.
The researchers found that Naia belonged to a mitochondrial lineage common to modern Native Americans and is found only in the Americas. The presence of a genetic marker indicates that the girl was maternally related to living Native Americans, and traces her ancestry to the same source population as contemporary Native Americans.
According to the researchers, the craniofacial divergences are probably the result of evolutionary changes that happened in Beringia over the last 9,000 years.
Shown is Naia's upper right third molar, which was used for both radiocarbon testing and DNA extraction.